5 Answers
5

There are a number of times when a Planetary Fortress is useful, but before we get to that let's look at what a PF is:

150M/150G

0 Additional Supply

40 AoE damage (20 dps)

Stationary

The biggest cost of a Planetary Fortress is the lack of building an Orbital Command. If OC wasn't so useful, there is little reason not to build the PF. It has heavy HP and Splash Damage, and doesn't add toward your supply max.

So when does the ability to have an extra Siege Tank worth of damage out weigh the benefit of Orbital Command? Well there is one situation where you have Command Centers in vulnerable positions that may not have quick reinforcements: Expansions. Actually if you watch a lot of high level matches, you'll start to notice that a 3rd or 4th expansion often becomes a Planetary Fortress. Since you're expanding past your natural by the 3rd, you often don't have your army in convenient position. As a result, Planetary Fortress helps to delay attackers and often repel harassment. Ever try to kill a PF with just Zerglings? it ain't pretty.

So next time you're thinking: do I really need a Planetary Fortress? ask yourself this: Do you really need that 3rd expansion to stay alive?

Another reason they're good for a 3rd/4th expansion is that once you have 2-3 Orbital Commands, you'll have a steady stream of MULE's and enough energy for a scan whenever you need one. This frees up future expansions to use Planetary Fortresses for less opportunity cost.
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WikwocketAug 31 '10 at 13:56

5

People also tend to build planetary fortresses on the gold mineral expansions.
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Lotus NotesAug 31 '10 at 17:12

1

@Carl as Gold expansions are usually more exposed I think that's definitely a good strategy
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tzenesAug 31 '10 at 17:19

They are quite usefull and if you watch some highlevel replays they are used quite often.

The best use is at hard to defend expansions where it can delay the enemy until your units arrive which can allow you to take more expansions than the enemy (but it's no replacement for troops).

Some people also place them at choke points where they add a lot of defense due to the high HP and quite good damage, but they are quite expensive, so you should have an expansion advantage before even thinking about using a command center for this instead for another expansion.

There is also the somewhat unorthodox cheese strategy "Planetary fortress rush". I would not recommend this because..well, it's just not that good to be honest :) I've never seen it used against me and I've never tried it myself.

+1, the Planetary fortress rush has been used against me in 4v4, by all 4 enemies at the same time. It was very well coordinated and nearly impossible to stop unless you're already prepared for it.
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Lotus NotesSep 3 '10 at 21:27

The epicness of a dual planetary fortress rush can be seen here: youtube.com/watch?v=R5NgS7UPsjc (game starts at the four minute mark, in case you are impatient)
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sthSep 6 '10 at 5:58

I successfully used PT on an expansion site yesterday. Playing agains a Terran and saw he was going to rush with MMM. The PT successfully blasted a good bit of his forces and held it's ground until reinforcements arrived. After that experience I am now a big fan of PT on exposed/vulnerable expansions